Understanding factors involved in the regulation of human hematopoiesis requires evaluation of the interaction between relatively homogeneous cell populations and pure growth factors. We are utilizing cell separation procedures to provide methods for obtaining markedly enriched cell populations from marrow wherein heterogeneous cells coexist. We have established in vitro clonogenic assays for detecting proliferation and differentiation of pluripotent (CFU-GEMM), erythroid (BFU-E), and granulocyte-monocyte (CFU-GM) human stem cells. Of particular utility has been the addition of immunologic techniques to separate selectively and recover antigenically defined hematopoietic cells and cells modulating their proliferation and differentiation. We will extend our use of immunologic "panning" (an antibody-mediated plate-binding technique) in combination with density and adherence separation procedures to provide relatively homogeneous cell populations for generating stimulatory or inhibitory growth substances and to provide target cells for these factors. In addition, the use of purified growth factors will permit more precise delineation of regulatory interactions, particularly those occurring within the marrow hematopoietic microenvironment. We plan specifically to extend our recent studies and investigate several critical biologic variables involved with human hematopoietic regulation: (1)\production of stimulatory and inhibitory hematopoietic growth factors by defined immunologic subsets; (2)\detection of production and characterization of a differentiation-inducing factor for granulopoiesis (termed GM-CSF-D, differentiation factor-DF, or MGI-2), separate from the previously well-defined proliferation-inducing factor (GM-CSF-P), by use of target cells capable of being induced into differentiating (the human leukemic cell line HL-60); (3)\the responsiveness of enriched marrow target cells to a variety of immune system modifiers (e.g., interleukins), other substances, and purified growth factors; and (4)\the effects of the "maturation-inducing" substances, retinoic acid and vitamin D, on altering production of and responsiveness to hematopoietic growth factors including stem cell self-replication. These studies will be performed with normal marrow cells to obtain physiologic baseline values and with marrow cells from patients with myeloid leukemia, pre-leukemia, other myeloproliferative and granulopoietic disorders to assess possible pathophysiological mechanisms in these diseases. (IS)